The flu virus lasts on average of one to two weeks and you're usually contagious the day before the onset of the flu and you'll remain contagious with the flu for up to 7 to 10 days after catching the flu.
Within a day or two of catching the flu virus you should start feeling the symptoms of the flu such as muscle pain, chills, dehydration, fatigue, fever, flushing, loss of appetite, body ache, or sweating.
Other symptoms of the flu are congestion, runny nose, sneezing, chest pressure, head congestion, headache, nausea, shortness of breath, sore throat, or swollen lymph nodes.
If you experience any of those symptoms of the flu then you should stay home from school or work until you're feeling better and are no longer contagious which is about 7 to 10 days after catching the flu virus.
You should be feeling better within one to two weeks of catching the flu.